1. Field of the Invention
The present invention relates to a wood club head having a face constituted by convex surfaces curved vertically and horizontally, and particularly to a wood club head having a devised face.
2. Description of the Related Art
In the background art, the face of a wood club head acting as a hitting surface is constituted by vertically and horizontally curved surfaces each having a radius of about 10 inches. The vertically and horizontally curved surfaces are called a roll and bulge respectively. Assume the case that hitting occurs when a face 2 of a head 1 is perpendicular to a target. In this case, if the face 2 is formed to be a flat surface as shown in FIG. 6, and if a ball B is hit at a sweet spot S of the face 2, which is an intersecting point between the face 2 and a perpendicular drawn from the center of gravity to the face 2, the ball B will be hit substantially straight toward the target. If the ball B is hit a little to a heel 3 side of the face 2 off of the sweet spot S, however, the head 1 swings counterclockwise around the center of gravity to thereby make the ball B1 rotate clockwise (slice spin) as shown by the ball B1 in FIG. 7. On the contrary, if the ball B is hit a little to a toe 4 side of the face 2, the head swings clockwise around the center of gravity to thereby make the ball B rotate counterclockwise (hook spin) as shown by the ball B2 in FIG. 7. Generally, such a spin generation effect is called a gear effect. That is, when a ball is hit a little to the right or to the left off the sweet spot S of the face 2, the ball is subjected to a slice or hook spin so that the ball is made to fly missing the target direction largely to the right or left and the directivity of the hit ball is remarkably reduced. To eliminate such a disadvantage, therefore, the face 2 is formed into a curved surface. Since the balls B1 and B2 fly out in the direction of the normal of the curved surface when a horizontally-curved surface is formed as the face 2 as shown in FIG. 8 even when the ball is hit a little off of the sweet spot S, the foregoing gear effect is canceled so that the directivity of the ball is improved. Further, such a phenomenon is generated also in the case where the ball is hit a little to the up or down side off the sweet spot S. In this case, the direction of hitting the ball is corrected by a so-called roll radius forming a vertically-curved surface and the flight distance of the hit ball is increased in place of the foregoing directivity of the ball. The horizontal bulge radius has been empirically selected to be about 10 inches for about ten years or more since a persimmon tree was selected to be used as a material of a head.
Recently, as the material of a wood club, however, titanium or an alloy thereof, which is a metal material having low specific gravity, high hardness, and high strength, has been frequently utilized, and the size of a head and the length of a club shaft have been increased. As a result, the inertial moment of the head is increased (for example, the value is increased from 10-20 gxc2x7mmxc2x7sec2 in the case of a persimmon head to 20 gxc2x7mmxc2x7sec2 or more in the case of a so-called metal head) so that, even in the case where a ball is hit at a portion a little off the center, that is, at a portion off the sweet spot S, a head is suppressed from swinging around the center of gravity and generation of the foregoing gear effect is reduced. As a result, there arises a problem that the value of the bulge radius which has been empirically taken into confidence is made unsuitable.
In order to eliminate the foregoing disadvantage, an object of the present invention is to provide a wood club head having a bulge radius which is suitable for a large and hollow head.
In order to achieve the above object, according to the present invention, there is provided a wood club head having a face constituted by convex surfaces curved vertically and horizontally, wherein a bulge radius forming the substantially horizontally-curved surface is set to be not smaller than 15 inches.